A scanning ophthalmoscope of the kind described above is disclosed in U.S. Pat. No. 4,886,351. This ophthalmoscope operates to examine the ocular fundus with a high resolution. For this purpose, the beam path of a point light source is deflected in a scan-like manner with the aid of a light-deflecting device and is imaged on the ocular fundus. The light reflected at the ocular fundus transilluminates the same light-deflecting device in the opposite direction and is thereafter deflected by a beam splitter into the viewing beam path.
A pinhole diaphragm is provided in the viewing beam path in a plane conjugated to the plane of the point light source to improve the contrast and the resolution. A detector detects the light transmitted through the pinhole diaphragm. The diameter of the pinhole diaphragm is so selected that only the light reflected at the ocular fundus reaches the detector. The output signal of the detector is conducted to an image memory and a monitor which is synchronized with the light-deflecting device and thereby provides a television image of the illuminated region of the eye.
The point light source can be realized by the beam waist of a focussed laser beam. A light-wave conductor can be mounted between the laser and the objective generating the waist of the laser beam for providing a physical separation between the laser and that part of the ophthalmoscope which is disposed in the proximity of the patient. In this way, a compact configuration of that part of the apparatus which is to be aligned to the eye of the patient is possible.
The light-deflecting device comprises a mirror-wheel scanner and a galvano scanner and, with this ophthalmoscope, the light-deflecting device generates electromagnetic stray fields. Because of the spatial proximity between the deflecting device and the detector, the electromagnetic stray fields can easily lead to interference signals at the output of the detector and in the signal processing downstream of the detector. Complex screening measures are required to prevent such interference signals.
The detector is a photomultiplier in ophthalmoscopes which are especially light sensitive. Since the photomultiplier is driven at a high voltage of approximately 1000 volts, very costly protective measures must be taken for reasons of apparatus safety. These protective measures furthermore require a great deal of space.
A very compact configuration of such an ophthalmoscope is on the other hand desirable since the person conducting an examination must align the ophthalmoscope with the eye of the patient in that the examining person looks at the eye of the patient laterally of the ophthalmoscope and views the light spot generated there by the ophthalmoscope. A compact configuration of the ophthalmoscope therefore makes possible a comfortable and rapid alignment of the ophthalmoscope with the eye of the patient.